(i) Field of the Invention
The invention relates to a device for regulating the flow of gases having substantially different molar masses.
(ii) Description of Related Art
In the field of the analysis of very-high purity gases, it is increasingly necessary to analyze sequentially various kinds of gases using the same analyzer, such as a trace-impurity analyzer like an atmospheric-pressure ionization mass spectrometer.
In order to ensure that the analyzer operates optimally, it is necessary to set the volume flow rate of the gas introduced into the analyzer at an approximately constant value irrespective of the gas to be analyzed.
In order not to contaminate the gas to be analyzed, calibrated orifices are used for regulating the gas flow rate, these orifices being mounted in a line for delivering gas to the analyzer. In the sonic regime, in which the pressure upstream of the orifice is at least twice as high as the pressure downstream of the orifice, controlling the pressure upstream of the orifice enables the flow of gas into the delivery line to be controlled within a certain range.
Nevertheless, regulating a gas flow using a calibrated orifice poses a problem in the case in which the gases to be analyzed have substantially different molar masses, such as, for example, hydrogen and nitrogen, since the volume flow rate is, for the same upstream pressure, proportional to M.sup.-1/2, where M is the molar mass of the gas.
In this case, the volume flow rate of hydrogen through a calibrated orifice is, for the same upstream pressure, approximately four times greater than the flow rate of nitrogen.
A large variation in the pressure upstream of the orifice in order to obtain the optimum flow rate is in most cases not possible for technical reasons. Either the plant cannot withstand the high pressure necessary for obtaining the same optimum flow rate for a gas of greater molar mass, in the case in which the orifice is designed for an optimum flow rate of a gas having a low molar mass, or, in the opposite case, the necessary upstream pressure is so low that sonic conditions, necessary for regulating the flow rate, are no longer guaranteed.
Consequently, an optimum flow rate of gas to be delivered to the analyzer can no longer be guaranteed without changing the pressure upstream of the orifice in significant amounts.
In addition, such large variations in the upstream pressure must be avoided in the field of the analysis of very-high purity gases, since these variations lead to transient regimes during which any surface in contact with the gas is likely to desorb and adsorb molecules, a process likely to modify the composition of the gases flowing in the lines.
A problem, similar to the one raised above, occurs for dilution units used with gases of substantially different molar mass.
A device designed for the feed of an analyzer having very high sensitivity is known from document FR-A-2714968 in the name of the Applicant Company. This device includes means for splitting an overall flow of a source of pure gas in order to feed various stages of a dilution unit with a precise partial flow. This flow splitting is produced using two bypassed lines connected in parallel to a line for sampling the pure gas. A calibrated orifice is placed at the inlet of each bypassed line and a flow regulator is placed in the sampling line in order to impose the overall flow rate feeding the dilution unit.
With an imposed volume flow rate, the pressure upstream of the orifice is proportional to M.sup.+1/2, where M is the molar mass of the gas flowing through the restriction. It is therefore understood that, at an imposed volume flow rate, the upstream pressure in the case of a light gas such as hydrogen is approximately four times lower than that in the case of nitrogen.
If the orifices for the flow splitting are therefore designed for a gas of high molar mass, it is no longer possible to guarantee the sonic conditions, or barely so, when the same flow-splitting means are used with a light gas. However, for precise regulation of the flow, it is absolutely essential to satisfy the conditions of a sonic or near-sonic regime. In this case too, the problem occurs of transient regimes affecting the composition of the gases in the lines.